They have the same ratio of carbon-14 to carbon-12 as the atmosphere, and this same ratio is then carried up the food chain all the way to apex predators, like sharks.
From this science, we are able to approximate the date at which the organism were living on Earth.
Once an organism dies, it stops taking in carbon-14.
Some of the carbon-14 might find its way into living creatures.
After a creature's death the isotope would slowly decay away over millennia at a fixed rate.
Here’s an example using the simplest atom, hydrogen. Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.
Radiocarbon dating uses isotopes of the element carbon. Cosmic rays – high energy particles from beyond the solar system – bombard Earth’s upper atmosphere continually, in the process creating the unstable carbon-14. Because it’s unstable, carbon-14 will eventually decay back to carbon-12 isotopes.
For other examples, see the essays on Temperatures from Fossil Shells and Arakawa's Computation Device.
The prodigious mobilization of science that produced nuclear weapons was so far-reaching that it revolutionized even the study of ancient climates.
Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.
Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere.
It can be used on objects as old as about 62,000 years.